1. Field of the Invention
The present invention relates to a process and an apparatus for producing microspheres of fissile and/or fertile materials.
2. Description of the Prior Art
Nuclear fuels in a spherical particulate form used in high temperature gas-cooled reactor (HTGR) and other types of nuclear reactors are conventionally produced by either the dry process such as powder metallurgical technology or the wet process such as ion exchange resin technique, hydrolysis (H process), sol-gel process and gel precipitation technique. The method using hydrolysis is also known as the internal gelling method, and in this method, a feed, solution prepared by dissolving hexamethylenetetramine or the like in an aqueous solution of a nuclear fuel material such as thorium, uranium or plutonium is divided into small droplets which are gelled by ammonia formed as a result of decomposition of the hexamethylenetetramine upon heating said droplets to elevated temperatures. The sol-gel process and gel precipitation technique are sometimes collectively referred to as the external gelling method. In the sol-gel process, a sol is used as the feed solution, and in the gel precipitation technique, an aqueous solution is used as the feed solution. In either method, ammonia and other gelling agents are caused to act externally on small droplets of the feed solution. In the conventional external gelling method, the resulting gel particles are finally recovered into ammonia water or other ammoniacal aqueous solutions. The gelling step consists of the following three stages:
(1) forming small droplets of the feed solution in a gaseous medium or organic solvent medium which has lower density than the ammoniacal aqueous solution for recovery and which forms a separate phase from said aqueous solution and letting the droplets fall by gravity;
(2) passing the droplets of a uniform size through an ammoniacal gaseous or organic solvent medium so that the shell or the outer skin of each droplet is sufficiently gelled to prevent its deformation at the time when it passes through the interface between the gaseous or organic solvent medium and the ammoniacal aqueous solution for recovery; and
(3) completing the gellation of the droplets in the ammoniacal aqueous solution.
The gelled particles are subsequently washed, dried and sintered to produce the final product. The sphericity of the final product generally depends on the sphericity of the gel particles. The greater the surface tension of the ungelled droplets that tends to contract them into a completely spherical form or the smaller the force that will deform the droplets into a non-spherical shape, the closer to complete sphericity the particles of the final product are.
If the feed solution is converted into small droplets and their shell or outer skin is gelled in a gaseous medium rather than in an organic solvent, the great surface tension at the interface with the feed solution ensures the production of small droplets having a higher degree of sphericity, but on the other hand, the droplets fall so rapidly that they receive a great impact when they enter the aqueous solution in the recovery step. Therefore, if the size of the droplets is not sufficiently small, they do not have sufficient time to provide a hard skin before they enter the aqueous solution, and the impact to which the droplets are subjected at the time they enter the aqueous solution makes it impossible to produce completely spherical particles.
In some cases, an organic solvent such as methyl isobutyl ketone is used as the medium in which the feed solution is converted to small droplets and their skin is gelled. However, organic solvents have small surface tension at the interface with water (ca. 10 dyne/cm for methyl isobutyl ketone), and if the size of the droplets is not sufficiently small, they have only a low degree of sphericity even before they are dropped into the gelling aqueous solution. If droplets about 1.7 mm in diameter prepared from a sol containing 1 mol of thorium per liter are gelled, particles of thorium oxide having a diameter of about 500 .mu.m are formed but their sphericity is about 1.1. Thorium oxide particles having a diameter of about 600 .mu.m prepared from droplets with a diameter of about 2.0 mm have a sphericity of about 1.2 and also cannot be described as spherical particles.
None of the organic solvents available that have a lower density than the aqueous solution for recovery are cheap enough to be used in great quantities. Furthermore, the ability of these solvents to dissolve ammonia and their surface tension at the interface with water are so small that they cannot be employed in the external gelling method. Therefore, it has been difficult for the existing external gelling techniques to produce large particles with a higher degree of sphericity.